1. Field of the Invention
This invention relates to a method for controlling a structure of a two-phase steel, and more particularly to a method for advantageously improving a steel structure in a good efficiency and a low cost by effectively utilizing a treatment through an application of a magnetic field in a production course of a two-phase steel.
2. Description of the Related Art
As a desirable property of a steel, it is required to have a high strength and a high toughness. In general, it is very difficult to simultaneously establish these two properties because the toughness of the steel lowers as the strength increases.
As means for establishing the two properties, there have hitherto been proposed a two-phase formation of the steel structure and a formation of fine crystal grains.
In the two-phase steel, the structure is very important for obtaining the desired properties, so that there are proposed various structure controlling methods.
In these conventional methods, however, satisfactory properties are not necessarily obtained, so that it is demanded to develop a new structure controlling method.
On the other hand, a method of conducting a high reduction rolling is used as means for forming fine grains, but there is already a limit in the improvement of the properties through a contrivance of the rolling method. Particularly, in case of the steel sheet, flat-shaped crystal grains increase in the rolling direction or a texture of aligning an orientation of the crystal grain in a certain direction develops, so that there are caused problems that an absorption ability of impact energy lowers, and the surface quality of the steel sheet is degraded, and the like.
For this end, it is desired to develop a new method for controlling the steel structure.
As one of such structure controlling methods, there is considered a method utilizing a magnetic field.
For example, Palmai Zoltan have reported (Gepgyartastechnologia. vol. 22(1982), page 463) that when a magnetic field of 0.57 T (T is a unit showing an intensity of a magnetic field: tesla) is applied during heat treatment for inversely transforming steel of Fe-0.60C-0.30Si-0.72Mn composition from martensite structure to austenite structure, ferrite phase is stabilized to increase residual ferrite amount as a study showing an effect of magnetic field to steel.
However, the above pioneering study is not industrially applied because the effect of the magnetic field is not clearly observed due to the weak magnetic field.
Moreover, there is not made a study or an invention on the structure control of steel utilizing the magnetic field up to the present time.
Since a superconducting magnet capable of applying a strong magnetic field has recently been developed, the inventors have made studies on the structure control of the steel utilizing the magnetic field.
As a result, the inventors have developed a method for controlling a structure of a two-phase steel wherein an inversely transformed austenite phase is aligned in a direction of an applied magnetic field by conducting an inverse transformation under heating in the magnetic field and disclosed in JP-A-11-315321, Bulletin of the Japan Institute of Metals, vol. 38, No. 5(1999), page 380 and Scripta materialia, vol. 32, (2000), page 499.
The invention lies in a further improvement of the above method and is to provide a method for controlling a structure of a two-phase steel wherein the control of two-phase structure can be made in a very short time by utilizing a treatment through application of a magnetic field during the structure control of the two-phase steel and the productivity can be increased by shortening the heating time and the cost can be reduced by decreasing the fuel cost for heating as compared with a case of controlling through usual heat treatment.
The inventors have made various studies in order to achieve the above object and discovered that when the steel is subjected to a work forming a true strain of not less than 0.1 prior to a heat treatment within a temperature range forming (xcex1xe2x88x92xcex3) two-phase zone or two-phase zone of xcex1-phase and xcex3-phase and then a magnetic field is applied in the heat treatment at such a two-phase zone, it is rendered into an aligned two-phase structure in the direction of the applied magnetic field in a very short time and such an alignment direction is determined only by the direction of applied magnetic field irrespectively of the previous forging direction, rolling direction or the like, and as a result, the invention has been accomplished.
That is, the invention lies in a method for controlling a structure of a two-phase steel, characterized in that a steel containing C: 0.05-0.80 mass % is subjected to a work forming a true strain of not less than 0.1 at a temperature zone of xcex1-phase or xcex3-phase and then a magnetic field of 0. 1-20 T is applied thereto within a temperature range forming a two-phase zone of xcex1-phase and xcex3-phase.